1. Field
The disclosed embodiments concern an electrically modulable and extended light source designed to inject excess carriers into a semiconductor wafer, thereby illuminating it. In particular, this source, by having a measurement device, makes it possible to characterize electrical properties of this semiconductor wafer, such as the volume lifetime and the rate of surface recombination of the minority carriers of a semiconductor wafer. The disclosed embodiments apply more particularly to the field of the photovoltaic industry and microelectronics.
2. Brief Description of Related Developments
New electronic components and the future generation of devices within microelectronic technology, and especially in the field of very large scale integration of integrated circuits, require starting materials of increasingly better purity, and the same is true of photovoltaic cells. Metals are primarily responsible for contamination of silicon wafers. The presence of these metals reduces the performance, reliability, and the ratio of these devices, that is, the number of “good” devices of a silicon wafer divided by the total number of devices on this same wafer. For example, in transistors, alkaline metals introduce a mobile charge in the thin oxide layer which then neutralizes the insulating properties of oxides. Moreover, the transition metals act like electron traps. Thus, a concentration of impurities greater than 1010/cm3 can render a silicon wafer totally unusable for the manufacture of certain devices.
Thus, contamination represents a relatively costly threat to the microelectronics industry and it is thus necessary to measure and control this contamination during the course of the integrated circuit manufacturing process.
Furthermore, while the threat is considered less critical to the photovoltaic industry, metal contamination can degrade the conversion yield of the cells and it can then exert an influence on the sales price of the end product.
The physico-chemical techniques traditionally used to measure the contamination are not applicable to a production line inspection. It would be necessary to extract aqueous solutions of metal contaminants from the wafer and then measure them by spectroscopic methods. These techniques are too long, too costly and destructive, and so not applicable to a use on the production line.
Techniques based on the measurement of the electrical effects of the contamination rather than the concentration of the impurities have been proposed, such as the technique of decrease in photoconductivity at ultrahigh frequency (micro-PCD). These techniques involve measuring the lifetime of the minority carriers τb in the semiconductor wafer, the lifetime of the minority carriers being a parameter connected to the concentration of impurities.
However, the measurement of this parameter τb is tied to the rate of surface recombination S, and it is not easy to extract its value directly. The alternative techniques proposed generally measure an effective lifetime τeff, which is at the same time related to τb and S. In order to be able to extract τb, it is thus necessary either to determine in very precise manner the value of S or to render the rate of recombination S negligible in relation to τb. Generally, the solution proposed consists in a supplementary step of passivation of the surface of the silicon wafers prior to the measurement step. However, this surface passivation step is not very adapted for use on the production line, since on the one hand it is liable to damage the wafer and, on the other hand, it causes an additional delay in the process of measurement and quality inspection of the wafer, thus leading to a further production cost.
The document J. Appl. Physic. Vol. 93, No 8 (2003) describes another alternative measurement technique based on measuring the microwave phase shift between the modulation signal of an exciting light source and the microwaves reflected by a semiconductor wafer so illuminated by the exciting light source. Contrary to the techniques previously mentioned, this technique makes it possible to independently access the volume lifetime of the minority carriers τb and the rates of surface recombinations S in the semiconductor wafer without having to proceed with a passivation of the surfaces of the wafer.